Cell Membrane Structure and Function PDF

Membrane Structure an Function... Figure 7.1 How do cell membrane proteins help KEY CONCEPTS regulate chemical traffic? 7,1 Cellular membranes are fluid mosaics of lipids...

Membrane Structure an Function... Figure 7.1 How do cell membrane proteins help KEY CONCEPTS regulate chemical traffic? 7,1 Cellular membranes are fluid mosaics of lipids channel for a stream of potassium ions (K+) to exit the cell at a and proteins precise moment after nerve stimulation. (The green ball in the 7.2 Membrane structure results in selective center represents one K+moving through the channel.) In this permeability case, the plasma membrane and its proteins not only act as an 7.3 Passive transport is diffusion of a substance outer boundary but also enable the cell to carry out its func· across a membrane with no energy investment tions. The same applies to the many varieties of internal mem- 7.4 Active transport uses energy to move solutes branes that partition the eukaryoticceH: The molecular makeup against their gradients of each membrane allows compartmentalized specialization in 7.5 Bulk transport across the plasma membrane cells. To understand how membranes work, we'll begin byex- occurs by exocytosis and endocytosis amining their architecture. r·ljjin '. life at the Edge r~:'I~~:~~;~branes are fluid mosaics of lipids and proteins he plasma membrane is the edge of life, the boundary T that separates the living cell from its surroundings. A remarkable film only about 8 om thick-it would take over 8,000 to equal the thickness of this page-the plasma Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abun- dant lipids in most membranes are phospholipids. The ability membrane controls traffic into and out of the cell it surrounds. of phospholipids to form membranes is inherent in their Like all biological membranes, the plasma membrane exhibits molecular structure. A phospholipid is an amphipathic mol- selective permeability; that is, it allows some substances to ecule, meaning it has both a hydrophilic region and a cross it more easily than others. One ofthe earliest episodes in hydrophobic region (see Figure 5.13). Other types of mem- the evolution of life may have been the formation of a mem- brane lipids are also amphipathic. Furthermore, most of the brane that enclosed a solution different from the surrounding proteins within membranes have both hydrophobic and hy- solution while still permitting the uptake of nutrients and drophilic regions. elimination of waste products. The ability of the cell to dis- How are phospholipids and proteins arranged in the mem- criminate in its chemical exchanges with its environment is branes of cells? You encountered the currently accepted model fundamental to life, and it is the plasma membrane and its for the arrangement of these molecules in Chapter 6 (see component molecules that make this selectivity possible. Figure 6.7). In this fluid mosaic model, the membrane is a fluid In this chapter, you will learn how cellular membranes con- structure with a ~mosaic" ofvarious proteins embedded in or at- trol the passage of substances. The image in Figure 7.1 shows tached to a double layer (bilayer) of phospholipids. Scientists the elegant structure ofa eukaryotic plasma membrane protein propose models as hypotheses, ,,',oays oforganizing and explain- that plays a crucial role in nerve cell signaling. This protein re- ing existing information. Well discuss the fluid mosaic model in stores the ability of the nerve cell to fire again by providing a detail, starting with the story of how it was developed. 125 Membrane Models: Scientific Inquiry they have hydrophobic regions as well as hydrophilic regions. If such proteins were layered on the surface ofthe membrane, their Scientists began building molecular models of the membrane hydrophobic parts would be in aqueous surroundings. decades before membranes were first seen with the electron In 1972, S.,. Singer and G. Nicolson proposed that mem- microscope in the 1950s. In 1915, membranes isolated from brane proteins are dispersed, individually inserted into the red blood cells were chemically analyzed and found to be phospholipid bilayer with their hydrophilic regions protruding composed of lipids and proteins. Ten years later, two Dutch (Figure 7.3). This molecular arrangement would maximize scientists, E. Gorter and F. Grendel, reasoned that cell mem- contact of hydrophilic regions of proteins and phospholipids branes must be phospholipid bilayers. Such a double layer of with water in the cytosol and extracellular fluid, while provid- molecules could exist as a stable bouodary between two aque- ing their hydrophobic parts with a nonaqueous environment. ous compartments because the molecular arrangement shel- In this fluid mosaic model, the membrane is a mosaic of pro- ters the hydrophobic tails of the phospholipids from water tein molecules bobbing in a fluid bilayer of phospholipids. while exposing the hydrophilic heads to water (Figure 7.2). A method of preparing cells for electron microscopy called Building on the idea that a phospholipid bilayer was the main freeze-fracture has demonstrated visually that proteins are in- fabric of a membrane, the next question was where the proteins deed embedded in the phospholipid bilayer of the membrane. were located. Although the heads of phospholipids are hydro- Freeze-fracture splits a membrane along the middle of the philic, the surface ofa membrane consisting of a pure phospho- phospholipid bilayer, somewhat like pulling apart a chunky lipid bilayer adheres less strongly to water than does the surface peanut butter sandwich. When the membrane layers are of a biological membrane. Given these data, Hugh Davson and viewed in the electron microscope, the interior of the bilayer James Danielli suggested in 1935 that this difference could be appears cobblestoned, with protein particles interspersed in a accounted for if the membrane were coated on both sides smooth matrix, as in the fluid mosaic model (Figure 7.4). with hydrophilic proteins. They proposed a sandwich model: a Some proteins travel with one layer or the other, like the peanut phospholipid bilayer between two layers of proteins. chunks in the sandwich. When researchers first used electron microscopes to study Because models are hypotheses, replacing one model of cells in the 19505, the pictures seemed to support the Davson- membrane structure with another does not imply that the orig- Danielli model.. By the 19605, the Davson-Danielli sandwich inal model.....as.....orthless. The acceptance or rejection of a had become....'idely accepted as the structure not only of the model depends on how well it fits observations and explains ex- plasma membrane but also ofall the cell's internal membranes. perimental results. A good model also makes predictions that By the end ofthat decade, however, many cell biologists recog- shape future research. Models inspire experiments, and few nized two problems with the model. The first problem 'was the models survive these tests without modification. New findings generalization that aU membranes of the cell are identical. may make a model obsolete; even then, it may not be totally \Vhereas the plasma membrane is 7-8 nm thick and has a scrapped, but revised to incorporate the new observations. The three-layered structure in electron micrographs, the inner fluid mosaic model is continuaJly being refined. For example, membrane of the mitochondrion is only 6 nm thick and looks recent research suggests that membranes may be ~more mosaic like a row of beads. Mitochondrial membranes also have a than fluid:' Often, multiple proteins semipermanently associate higher percentage ofproteins and different kinds in specialized patches, where they carry out common func- of phospholipids and other lipids. In short, 1i tions. Also, the membrane may be much more packed with membranes with different functions differ in proteins than imagined in the classic fluid mosaic model. Let's chemical composition and structure. now take a closer look at membrane structure. A second, more serious problem with the sand....'ich model was the protein placement. Unlike proteins dissolved in the cytosol, mem- brane proteins are not ''6)' soluble in...."J.ter, because they are amphilXlthic. that is, WATER Phospholipid bilayer HydrophobIC regions WATER of protein Figure 7.2 Phospholipid bilayer (cross section). Figure 7.3 The fluid mosaic model for membranes. 126 UNIT TWO The Cell  FI~7.4 Freeze-Fracture APPLICATION A cell membrane can be split Into its two lay- ers. re~ealing the ultrastructure of the membrane's interior. TECHNIQUE A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a mem- brane, splitting the phospholipid bilayer into two separated lay- ers. The membrane proteins go wholly With one of the layers (a) Movement of phospholipids. Lipids move laterally in a membrane. but flip-flopping across the membrane is quite rare, Fluid Viscous Plasma membrane Cytoplasmic layer Unsaturated hydrocarbon Saturated hydro- RESULTS These SEMs tails with kinks carbon tails show membrane proteins ", ' (bl Membrane fluidity. Unsaturated hydrocarbon tails of phospholipids (the "bumps") in the two have kinks that keep the molecules from packing together. layers, demonstrating that enhanCing membrane fluidity, proteins are embedded in the Inside of extracellular layer phospholipid bilayer, Inside of cytoplasmic layer Cholesterol The Fluidity of Membranes (cl Cholesterol within the animal cell membrane. Cholesterol Membranes are not static sheets of molecules locked rigidly in reduces membrane fluidity at moderate temperatures by reducing place. A membrane is held together primarily by hydrophobic in- phospholipid movement. but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids, teractions, which are much weaker than covalent bonds (see Figure 5.21). Mostofthe lipids and some ofthe proteins can shift... Figure 7.5 The fluidity of membranes. about laterally-that is, in the plane ofthe membrane, like party- goers elbo\\ingtheirwaythrough acrowded room (Figure 7.Sa). It is quite rare, however, for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to A membrane remains fluid as temperature decreases until the other; to do so, the hydrophilic part of the molecule must finally the phospholipids settle into a closely packed arrange- cross the hydrophobic core of the membrane. ment and the membrane solidifies, much as bacon grease The lateral movement ofphospholipids within the membrane forms lard when it cools. The temperature at which a mem- is rapid. Adjacent phospholipids switch positions about 107 times brane solidifies depends on the types of lipids it is made of. per second, which means that a phospholipid can travel about 2 The membrane remains fluid to a lower temperature if it is 11m-the length ofmany bacterial cells-in 1second. Proteinsare rich in phospholipids with unsaturated hydrocarbon tails (see much larger than lipids and move more slowly, but some mem- Figures 5.12 and 5.13). Because of kinks in the tails where dou- brane proteins do drift, as shown in aclassic experiment by David ble bonds are located, unsaturated hydrocarbon tails cannot Frye and Michael Edidin (Figure 7.6, on the next page). And pack together as closely as saturated hydrocarbon tails, and some membrane proteins seem to move in a highly directed this makes the membrane more fluid (Figure 7.5b). manner, perhaps driven along cytoskeletal fibers by motor pro- The steroid cholesterol, which is wedged between phospho- teins connected to the membrane proteins' cytoplasmic regions. lipid molecules in the plasma membranes of animal cells, has However, many other membrane proteins seem to be held virtu- different effects on membrane fluidity at different temperatures ally immobile by their attachment to the cytoskeleton. (Figure 7.5e). At relatively higher temperatures-at 37C, the CIl... PTH SEVEN Membrane Structure and Function 127  body temperature of humans, for example-cholesterol makes · In ui the membrane less fluid by restraining phospholipid movement. Do membrane proteins move? However, because cholesterol also hinders the close packing of EXPERIMENT phospholipids, it lowers the temperature required for the mem~ David Frye and MIChael Edidin, at Johns Hopkins University, labeled the plasma membrane proteins of a moose (ell brane to solidify. Thus, cholesterol can be thought of as a "tern· and a human cell with two different markers and fused the cells. perature buffer~ for the membrane, resisting changes in Using a microscope, they observed the markers on the hybrid cell. membrane fluidity that can be caused by changes in temperature. RESULTS Membranes must be fluid to work properly; they are usually about as fluid as salad oil. \Vhen a membrane solidifies, its per- meability changes, and enzymatic proteins in the membrane may become inactive-for example, if their activity requires Mixed proteins after 1 hour them to be able to move laterally in the membrane. The lipid Human cell composition of cell membranes can change as an adjustment Hybrid cell to changing temperature. For instance, in many plants that toler- CONCLUSION The mixing of the mouse and human mem- ate extreme cold, such as winter wheat, the percentage of un~ brane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. saturated phospholipids increases in autumn, an adaptation that keeps the membranes from solidifying during winter. SOURCE l. D, FI)'l! and M Edidin, The rapid i:'ltem1il:lng of ~II surfoce antlgl'flS after fCJm'l